Temperature control apparatus

ABSTRACT

A temperature control apparatus comprising an electric heater means, switching means connected between the heater means and an AC electric power source, and a thermo-detector means thermocoupled with the heater means to turn on and off the switching means below and above a predetermined temperature, in which a thyristor is employed for the switching means, the thermodetector means being a transformer which has a primary winding connected to the power source through a resistor means, a secondary winding connected to the control terminal of the thyristor and a core of magnetic materials having a Curie point of the predetermined temperature, a low coercive force and a high differential permeability at a magnetic field strength equal to the coercive force. Outputs on the transformer secondary winding fire the thyristor at each zero voltage of the AC voltage to permit the heater means to be excited below the predetermined temperature. No output is present on the secondary winding above the predetermined temperature so that the thyristor is not fired. An inductor having a winding and a core of similar magnetic materials as above may be substituted for the transformer to perform the same purpose, in which case the self-induced voltage on the inductor winding is coupled to the control terminal of the thyristor.

I United States Patent [191 Endo et al.

[ Nov. 18, 1975 TEMPERATURE CONTROL APPARATUS [75] Inventors: Masanori Endo; Ken Miyagi, both of Yokohama, Japan [73] Assignee: Tohoku Metal Industries Limited,

Sendai, Japan [22] Filed: June 10, 1974 [21] Appl. No: 478,093

[30] Foreign Application Priority Data June 11, 1973 Japan 48-67820[U] June 11, 1973 Japan 48-67821[U] Oct. 11, 1973 Japan 48-115742 [52] US. Cl. 219/503 [51] Int. Cl. H05B 1/02 [58] Field of Search 219/482, 494, 495, 501, 219/503; 323/89 R,'89 T, 36, 39

[56] References Cited UNITED STATES PATENTS 3,328,561 6/1967 Sakamoto et al. 219/495 3,369,108 2/1968 Strachan 219/501 3,427,436 2/1969 Finnegan 219/501 3,569,673 3/1971 Clark 219/501 Primary Examiner-R. N. Envall, Jr. Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-Flynn & Frishauf [57] ABSTRACT A temperature control apparatus comprising an electric heater means, switching means connected between the heater means and an AC electric power source, and a thermo-detector means thermo-coupled with the heater means to turn on and off the switching means below and above a predetermined temperature, in which a thyristor is employed for the switching means, the thermo-detector means being a transformer which has a primary winding connected to the power source through a resistor means, a secondary winding connected to the control terminal of the thyristor and a core of magnetic materials having a Curie point of the predetermined temperature, a low coercive force and a high differential permeability at a magnetic field strength equal to the coercive force. Outputs on the transformer secondary winding fire the thyristor at each zero voltage of the AC voltage to permit the heater means to be excited below the predetermined temperature. No output is present on the secondary winding above the predetermined temperature so that the thyristor is not fired.

An inductor having a winding and a core of similar magnetic materials as above may be substituted for the transformer to perform the same purpose, in which case the self-induced voltage on the inductor winding is coupled to the control terminal of the thyristor.

14 Claims, 11 Drawing Figures Sheet 1 of 2 3,920,956

U.S. Patsnt Nov. 18, 1975 TEMPERATURE CONTROL APPARATUS BACKGROUND OF THE INVENTION This invention relates to temperature control apparatus wherein the Curie point of an element of the apparatus is a factor in the temperature control thereof and, more particularly, to such apparatus wherein the excitation of an electric heater means by an AC power source is started and stopped at zero-voltage crossings of the AC voltage under the temperature control.

There have been known in the prior art several temperature control apparatus in which the Curie point is a factor in the temperature control.

A temperature operated switch consisting of a reed switch, permanent magnets and temperature sensitive magnetic materials having a Curie point of a predetermined temperature, has been used to sense a temperature and to control the power supply to a heater means. The operation of the heater means can be initiated when the temperature of the magnetic materials becomes below the Curie point, and it can be stopped when the temperature becomes above the Curie point. But temperature control using such a temperature operated switch encounters difficulty caused by disadvantages of theswitch, that is, the wear of contacts of the switch, a short life time of the switch and the resultant reduction in reliability. Furthermore, with such devices, itis not possible to start or stop the excitation of the heater means as a function of a zero voltage of the AC voltage from the power source.

Another known apparatus employs a transformer comprising a primary winding, a secondary winding and a core having a Curie point of a predetermined temperature with AC power being supplied to the primary winding and with a heater means being connected to the secondary winding. At a temperature above the Curie point, the voltage induced from primary winding to the secondary winding is reduced so that the operation of the heater means may be stopped. The apparatus requires theuse of a transformer of a great volume to obtain high calorie or energy transfer and has, therefore, an inferior temperature response. Furthermore, the known apparatus cannot achieve start and stop of the excitation of the heater means from zero' voltage portions of: the AC voltage from the power source.

There has been known another apparatus in which a 'thyristor such as an SCR or a Triac is connected between a heater element and an AC power source, a transformer having a core of the Curie point of a prede- -.termined temperature being provided, a primary winding being supplied with an input pulse train, and a secondary winding being coupled to a control terminal. At a temperature below the Curie point the pulses induced from the primary winding to the secondary winding fires the thyristor so that the heater means may be excited, and at a temperature above the Curie point no pulse is induced tothe secondary winding so that the heater means may not be excited. In this known apparatus, a pulse generator is required to obtain the pulse train, which complicates electric circuit of the apparatus.

ture control apparatus in which the excitation of a heater means may be started and stopped from the zero voltage of the AC voltage. I

Another object of this invention is to provide a temperature control apparatus having a long life time and high reliability.

Still another object of this invention is to realize the above objects by the use of circuitry elements of small volume.

Another object of this invention is to provide a temperature control apparatus comprising electric heater means, a thyristor, a transformer and a resistor in which the excitation of the heater means is started from zero voltage portions of 'the AC voltage.

Yet another object of this invention is to provide a temperature control apparatus comprising an electric heater means, a thyristor, an inductor and a resistor, in which the excitation of the heater means is started from zero voltage portions of the AC voltage.

A further object of this invention is to provide a transformer and an inductor which provide excellent temperature response to realize the above objects.

A temperature control apparatus of this invention comprises electric heater means excited by an AC electric power source, a thyristor, such as an SCR, SSS, Triac or the like, connected between the AC electric power source and the electric heater means for switching on or off the power supply to the electric heater means, a resistor, and a transformer which is thermocoupled with the electric heater means, the transformer comprising a primary winding, a secondary winding and a core having a cutie point of a predetermined temperature and a generally rectangular hysteresis loop. The primary winding is connected to the AC electric power source through the resistor and the secondary winding is coupled to the control terminal of the thyristor, whereby the thyristor may be turned on by the impulses present on the secondary winding.

Another temperature control apparatus of this invention comprises electric heater means excited by an AC electric power source, a thyristor connected between the AC electric power source and the electric heater means for switching on or off the power supply to the electric heater means, a resistor, and an inductor which is thermo-coupled with the electric heater means, the inductor comprising a winding and a core having a Curie point of a predetermined temperature and a generally rectangular hysteresis loop. The inductor winding is connected to the AC electric power source through the resistor and is coupled to the control terminal of the thyristor, whereby the thyristor may be turned on by the impulses self-induced on the winding.

Preferably, the core of the transformer or the inductor used in the apparatus of this invention ismade of magnetic materials having properties of a less coercive forceiand a high differential permeability at a magnetic field strength equal to the coercive force, besides the above described properties.

Further features and objects of this invention will become apparent from the following descriptions of embodiments of this invention with reference to the annexed drawings.

BRIEF'DESCRIPTION OF THE DRAWINGS FIG. 1a shows a circuit diagram of an embodiment of this invention,

FIG. 1b graphically illustrates the hysteresis loop of the core of the transformer in FIG. la,

FIG. 10 graphically illustrates the temperature response of the differential permeability at the coercive force of the core of the transformer in FIG. la,

FIG. 1d schematically illustrates voltage and current waveforms at various points in the circuit in FIG. 1a, when the heater is excited,

FIG. 1e schematically illustrates voltage and current waveforms at various points in the circuit in FIG. la, when the excitation of the heater is stopped,

FIG. 2 shows a circuit diagram of another embodiment of this invention,

FIG. 3a shows a circuit diagram of still another embodiment of this invention,

FIG. 3b graphically illustrates a temperature control of the voltage input to the gate of the thyristor in FIG.

FIGS.-4 and 5 show circuit diagrams of further embodiments of this invention, and

FIG. 6 shows a sectional view of an embodiment of a core of the transformer or the inductor used in an apparatus of this invention.

DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS Referring to FIG. 1a, an embodiment of this invention comprises an electric heater element 2 and a Triac 3 connected in series to an AC electric power source 1. A primary winding 7 of a transformer S is connected to the'AC electric power source through a resistor 4 and a secondary winding 8 of the transformer 5 is coupled to the gate of the Triac 3.

The transformer 5 is contained and positioned in an oven 9 in which the heater 2 is mounted so that the transformer is exposed to the temperature in the oven.

The core 6 of the transformer 5 is of a magnetic material, the Curie point of which corresponds to the temperature in the oven 9 to be maintained. The Core 6 exhibits a generally rectangular hysteresis loop with a less coercive force as shown in FIG. 1b and such a temperature response of the differential permeability miif at a magnetic field strength equal to the coercive force as shown in FIG. 1c in which the differential permeability udif gradually decrease slightly by elevation of the temperature and is rapidly reduced close to the Curie point Tc. Accordingly, the core 6 has ferr'o-magnetic properties below the Curie point and exhibits paramagnetism at and above the Curie point.

Examples of magnetic. materials exhibiting magnetic properties illustrated in FIGS. 1b and 1c and above described, are:

Example A magnetic material comprising Fe O 50.6-54.8 mol MnO 6.420.4 mol ZnO 43.0-24.8 mol and addition if necessary. Curie point: from 30 to 150C.

Example B magnetic material comprising Fe O 50.0 mol ZnO 33.0-38.0 mol NiO 12.0-17.0 mol and, if necessary, additions Curie point: from 0 to 120C Example C magnetic material comprising Ni 78-80 mol Nb, Cu, Mo and Fe Curie point: 395C I In operation, an Ac current i flows through the resistor 4 and the primary winding 7 from the AC electric power source, the AC voltage v of which exhibits, for example, a sine-curved waveform as shown at (l) in FIG. 1d.

Assuming that the AC voltage v is expressed by the equation of v Em sin 0), the resistance of the resistor 4 being R, and the inductance of the primary winding 7 being L, the AC current i is expressed as follows:

where, dz tan The phase angle 4) is ignored by realizing the inequality wL R, so that the AC current flowing through the resistor 4 and the primary winding 7 is in phase with the AC voltage as shown at (2) in FIG. 2d.

During a period when the temperature in the oven 9 and, therefore, of the core 6 of the transformer 5, is below the Curie point of the core, the core 6 has ferromagnetic properties as shown in FIG. 1b, so that the AC magnetic flux flows through the core 6. The.

waveform of the AC magnetic flux is a generally square waveform as shown at (3) in FIG. 1d because the core has a rectangular hysteresis loop with a less coercive force. Accordingly an impusle is induced and is present on the secondary winding 8 at each time when the AC voltage is zero voltage, with two successive impulses being of reverse polarity in correspondence with the AC voltage, as shown (4) inFIG. 1d.

The impulses v present on the secondary winding 8 are coupled to the gate of the Triac 3 to fire the Triac at each time when the AC voltage v is zero voltage to permit a current to flow through the Triac 3. As a result, the AC current i flows through the heater element 2, which is, in turn, excited to elevate the inner temperature of the oven 9.

When the inner temperature of the oven 9 and, therefore, the temperature of the core 6 is elevated to the temperature of the Curie point of the core 6, the phenomena of the ferro-magnetism of the core 6 disappear and the core 6 exhibits the para-magnetism. Thus, the magnetic flux does not almost flow through the core 6 as shown at (3) in FIG. 1e and impulses, therefore, are not induced on the secondary winding 8 enough to fire the Triac 3. Accordingly, the Triac 3 is not turned on and does not permit any current to flow therethrough. Thus the heater element 2 is not excited and the inne temperature of the oven 9 is lowered.

When the inner temperature of the oven 9 becomes below the temperature of the Curie point of the core 6, the ferro-magnetism of the core 6 recovers so that similar operation is again performed as'described in connection with FIG. 1d to elevate the inner temperature of the oven 9.

By the repetition of different of operations as above described with reference to FIGS. 1d an Ie, the inner temperature of the oven 9 is controlled to be substantially constant.

It will be understood that the excitation of the heater element 2 is started and stopped at the zero voltage of the AC voltage from the AC electric power source, because the Triac 3 connected between the heater element 2 and the power source is controlled in its turning on and off operation by impulses presented at every time when the AC voltage, is at its zero voltage level. Accordingly, the electric power is efficiently used without power loss.

It should be noted that the temperature control apparatus of this embodiment is simple, has a long life and has high reliability, because it consists of merely a heater element, a Triac, a transformer and a resistor.

Referring to FIG. 2, another embodiment of this invention shown in FIG. 2 is similar to the embodiment shown in FIG. 1a, except that an SCR is substituted for the Triac 3 with a diode 11 being connected in parallel with and in reverse polarity with the SCR. Similar parts are denoted by same numerals in FIG. 1a.

The operation of this embodiment will be easily understood to be similar as that of the embodiment in FIG. 1a, except that a half-wave current rectified by the diode l 1 flows through the heater element 2 even if the inner temperature of the oven 9 is at or above the Curie point of the core 6.

According to this invention, the inner temperature of the oven 9 is prevented from rapidly dropping from the temperature above the Curie point of the core 6 so that the damage of the oven and various parts in the oven due to the rapid temperature variation may be prevented. Furthermore the temperature control is performed smoothly and the ripple of the temperature is reduced.

. If the diode 11 is omitted from the embodiment in FIG. 2, the heater element 2 is not excited when the inner temperature of the oven 9 is at or above the Curie point of the core 6. But when the inner temperature of the oven 9 is below the Curie point of the core 6, impulses present on the secondary winding 8 fire the SCR 10 at every one period internal of the AC voltage, to permit the Current to flow through the heater element 2 during half periods when the AC voltage is applied to the SCR in the forward direction thereof. Accordingly, the apparatus having a circuit formation wherein the diode 11 of the embodiment in FIG. 2 is omitted can, also, perform temperature control.

FIG. 3a shows a circuit diagram of another embodiment which is 'a modification of the embodiment in FIG. 1a.

This embodiment is different from the embodiment in FIG. 1a, merely in that the voltage on the secondary winding 8 of the transformer 5 is supplied through the voltage divider or the potentiometer comprising a variable resistor 12 and a fixed resistor 13.

It will be noted from above description in connection with the embodiment in FIG. la that the heater element 2 is not excited during the period when the'inner temperature of the oven 9 is at or above the Curie point of the core 6.

During the period when the inner temperature of the oven 9 is below the Curie temperature of the core 6, impulses Vp are induced to the secondary winding 8, as shown at (4) in FIG. 1d. The impulses are voltage divided at the potentiometer before it is applied to the gate of the Triac 3. In this connection, thevoltage applied to the gate of the Triac 3 is controlled by regulating the variable resistor 12.

The voltage applied to the gate changes by elevation of the temperature of the core 6 as shown in FIG. 3b, in which the curve a shows the variation of the gate voltage when the resistance of the variable resistor 12 is minimum, the curve b showing the variation of the gate voltage at a maximum resistance of the resistor 12. The

voltage level V shown by a dotted line is a critical gate voltage at which the Triac 3 should be fired. It will be understoodfrom FIG. 3b that the maximum temperature at which the Triac Scan be fired is changed by the variation of the gate voltage of the Triac 3 from s temperature Tb to the temperature Ta. Accordingly, this embodiment has an advantage that the inner temperature of the oven 9 to be kept constant can be var- 6 ied only by regulating the resistance of the variable resistor in the potentiometer.

Another embodiment of this invention shown in FIG. 4 is similar as the embodiment shown in FIG. la except that the series circuit including the resistor 4 and the primary winding 7 is connected to the power source 1 through the heating element 2 so that the heater element 2 is applied the AC current through the series circuit even if the inner temperature of the oven 9 is at or above the Curie point of the core 6.

FIG. 5 shows an embodiment in which an inductor 14 is used in place of the transformer 5 in the above embodiments.

The inductor 14 comprises a winding 15 and a core 16 which has similar magnetic properties as the core 6 of the transformer 5 used in the above embodiments.

The winding 15 is connected to the AC power source 1 through the series resistor 4 and, also, is connected to the gate of the Triac 3.

In this embodiment impulses self-induced on the winding 15 are utilized to fire the Triac 3.

The operation of this embodiment will be easily understood to be similar as the embodiment in FIG. 1a.

This embodiment has an avdvantage that the simplification of the circuitry is realized more than the apparatus using a transfomer because the secondary winding is not necessary.

In connection with this embodiment, similar modifications will be understood to be also applied as shown in FIGS. 2 and 3a.

The core used in the transformer or the inductor in this invention may have, of course, any usual construction known in the prior art.

FIG. 6 shows a preferably construction of the core, in which the core comprising a U-shape magnetic piece 61 and a bar-type magnetic piece 62, both being connected and fixed to each other in such fashion that a circular closed loop of magnetic flux path may be formed.

The bar-type magnetic piece 62 is of the magnetic material having the Curie point equal to the temperature to be maintained in the temperature control apparatus, while the U-shaped piece 61 is of the magnetic material having the Curie point-higher than the Curie point of the bar-type magnetic piece 62.

A winding or windings are mounted on the U-shape piece 62 as shown by dotted lines in FIG. 6.

In the apparatus of this invention using the core in FIG. 6, the bar-type piece 62 serves as a sensor of the temperature, so that temperature control with excellent temperature response may be performed because the sensor is small in volume and, therefore, in heat capacity.

Furthermore, the .controlled temperature may be easily changed only by replacing the bar-type magnetic piece 62 by one having a different Curie point.

We claim:

1. A temperature control apparatus comprising:

a source of AC electric power,

electric heater means excited by said AC electric power source,

a thyristor connected between said AC electric power source and said electric heater means for switching on or off the power supply to said electric heater means,

' a resistor, and

a transformer thermo-coupled with said electric heater means, said transformer comprising:

i in I a primary winding coupled to said AC electric power source through said resistor,

v a secondary winding coupled to the control terrninal of said thyristor, and

a core on which said windings are wound, said core having a Curie point of a predetermined temperature, and said core having a generally rectangular hysteresis loop with a less coercive force and a high differential permeability at the magnetic field strength equal to the coercive force, whereby when said core is at temperatures below said predetermined temperature an impulse signal is present on said secondary winding at each zero voltage point of the AC power signal and when said coreis at a temperature at or above said predetermined temperature no output is present on said secondary winding,

said thyristor being turned on by the impulse present on said secondary winding when the temperature of said core is below said predetermined temperature to couple the AC electric power to said electric heater to energize said electric heater, said thyristor being maintained in its off condition when said temperature of said core is at or above said predetermined temperature. 2. The temperature control apparatus as claimed in claim 1, in which the core of said transformer is of one selected from the group of an alloy comprising 78-80 Ni, Nb, Cu, Mo and Fe, an alloy comprising 50.6-54.8 mol Fe O 6.420.4 mol MnO v24.8-43.0 mol ZnO and additions, and an alloy comprising l2.0-l7.0 mol NiO, 33.038.0 mol ZnO, 50.0 mol Fe O and additions.

3. The temperature control apparatus as claimed in claim 1, in which the core of said transformer comprises a bar-type magnetic piece having a Curie point of said predetermined temperature, and a generally V- shaped magnetic piece having a Curie point higher than the Curie point of said bar-type magnetic piece, both of said magnetic pieces being connected and fixed to each other such that a circular closed magnetic flux loop may be formed, and said windings of said transformer are mounted on said generally U-shaped magnetic piece.

4. The temperature control device as claimed in claim 1, in which said thyristor is an SCR, and further including a rectifying diode connected in parallel with, and in the reverse direction of, said SCR.

5. The temperature control device as claimed in claim 1, further comprising a variable voltage dividing means coupled to said secondary winding and coupling a portion of the output on said secondary winding to the control terminal of said thyristor. I

6. The temperature control device as claimed in claim 1, in which said primary winding and said resistor are connected in series, the series connection being connected to said AC electric power source through said electric heater means.

7. The temperature control device as claimed in claim 1, wherein the primary winding has an inductance L and said resistor has a resistance R (ohms), and

wherein the values of L and R are chosen such that:

wL R.

8. A temperature control apparatus comprising:

a source of AC electric power,

electric heater means excited by said AC electric power source,

a thyristor connected between said AC electric power source and said electric heater means for switching on or off the power supply to said electric heater means,

a resistor, and

an inductor thermo-coupled with said electric heater means, said inductor comprising:

a winding coupled to said AC electric power source through said resistor, and said winding being further coupled to the control terminal of said thyristor, and

a core on which said winding is wound, said core having a Curie point of a predetermined temperature, and said core having a generally rectangu lar hystersis loop with a less coercive force and a high differential permeability at the magnetic field strength equal to the coercive force, whereby when said core is at temperatures below said predetermined temperature a self-induced impulse signal is present on said winding at each zero voltage point of the AC power signal and when said core is at a temperature at or above said predetermined temperature no output is present on said winding,

said thyristor being turned on by the impulse present on said winding when the temperature of said core is below said predetermined temperature to couple the AC electric power to said electric heater to energize' said electric heater, said thyristor. being maintained in its off condition when said temperature of said core is at or above said predetermined temperature.

9. The temperature control apparatus as claimed in claim 8, in which the core of said inductor is of one selected from the group of an alloy comprising 78-80 Ni, Nb, Cu, Mo and Fe, an alloy comprising 50.6-54.8 mol Fe O 6.4-20.4 mol MnO 24.8-43.0 mol ZnO and additions, and an alloy comprising 12.0-17.0 mol NiO, 33.0-38.0 mol ZnO, 50.0 mol Fe O and additions.

10. The temperature control apparatus as claimed in claim 8, in which the core of said inductor comprises a bar-type magnetic piece having a Curie point of said predetermined temperature, and a generally U-shaped magnetic piece having a Curie point higher than the Curie point of said bar-type magnetic piece, both of said magnetic pieces being connected and fixed to each other such that a circular closed magnetic flux loop may be formed, and said winding of said inductor is mounted on said generally U-shaped magnetic piece.

11. The temperature control device as claimed in claim 8, in which said thyristor is an SCR, and further including a rectifying diode connected in parallel with, and in the reverse direction of, said SCR.

12. The temperature control device as claimed in claim 8, further comprising a variable voltage dividing means coupled to said winding and coupling a portion of the output on said winding to the control terminal of said thyristor.

values of L and R are chosen such that:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,920,956

DATED November 18, I975 IN'VENTOMS) I Masanori ENDO et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the patent, change the date of the last Japanese priority application from "October 11, 1973" to --October 17, 1973--;

Column 3, line 51, change "addition" to additions--;

line 66, change "w" to -wt-;

Column 6, line 34, change "preferably" to -preferable--;

Column 7, line 37, after "generally" change "V-" to Signed and Scaled this VISEAL] ninth y f March 1976 Attest.

RUTH Atlestin (ZZ' MARSHALL ANN g fiver (ommissinner oj'Parents and Tr d UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT N0. 1 3,920,956 DATED November 18, I975 INVENTOMS) I -Masanori ENDO et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: G

In the heading of the patent, change the date of the last Japanese priority application from "October 11, 1973" to --October 17, 1973; Q

Colunm 3, line 51, change "addition" to -additions-;

line 66, change "w" to -wt;

Q Column 6, line 34, change "preferably" to -preferable--;

Column 7, line 37, after "generally" change "V" to q glgnfld and Scaled this [SEAL] D of March 1976 Arrest: O

RUTH C. Mrh-SON C. MARSHALL DANN I nesting Officer ".f'latents and Trademarks 

1. A temperature control apparatus comprising: a source of AC electric power, electric heater means excited by said AC electric power source, a thyristor connected between said AC electric power source and said electric heater means for switching on or off the power supply to said electric heater means, a resistor, and a transformer thermo-coupled with said electric heater means, said transformer comprising: a primary winding coupled to said AC electric power source through said resistor, a secondary winding coupled to the control terminal of said thyristor, and a core on which said windings are wound, said core having a Curie point of a predetermined temperature, and said core having a generally rectangular hysteresis loop with a less coercive force and a high differential permeability at the magnetic field strength equal to the coercive force, whereby when said core is at temperatures below said predetermined temperature an impulse signal is present on said secondary winding at each zero voltage point of the AC power signal and when said core is at a temperature at or above said predetermined temperature no output is present on said secondary winding, said thyristor being turned on by the impulse present on said secondary winding when the temperature of said core is below said predetermined temperature to couple the AC electric power to said electric heater to energize said electric heater, said thyristor being maintained in its off condition when said temperature of said core is at or above said predetermined temperature.
 2. The temperature control apparatus as claimed in claim 1, in which the core of said transformer is of one selected from the group of an alloy comprising 78-80 % Ni, Nb, Cu, Mo and Fe, an alloy comprising 50.6-54.8 mol % Fe2O3, 6.4-20.4 mol % MnO2, 24.8-43.0 % mol ZnO and additions, and an alloy comprising 12.0-17.0 mol % NiO, 33.0-38.0 mol % ZnO, 50.0 mol % Fe2O3 and additions.
 3. The temperature control apparatus as claimed in claim 1, in which the core of said transformer comprises a bar-type magnetic piece having a Curie point of said predetermined temperature, and a generally V-shaped magnetic piece having a Curie point higher than the Curie point of said bar-type magnetic piece, both of said magnetic pieces being connected and fixed to each other such that a circular closed magnetic flux loop may be formed, and said windings of said transformer are mounted on said generally U-shaped magnetic piece.
 4. The temperature control device as claimed in claim 1, in which said thyristor is an SCR, and further including a rectifying diode connected in parallel with, and in the reverse direction of, said SCR.
 5. The temperature control device as claimed in claim 1, further comprising a variable voltage dividing means coupled to said secondary winding and coupling a portion of the output on said secondary winding to the control terminal of said thyristor.
 6. The temperature control device as claimed in claim 1, in which said primary winding and said resistor are connected in series, the series connection being connected to said AC electric power source through said electric heater means.
 7. The temperature control device as claimed in claim 1, wherein the primary winding has an inductance L and said resistor has a resistance R (ohms), and wherein the values of L and R are chosen such that: omega L << R.
 8. A temperature control apparatus comprising: a source of AC electric power, electric heater means excited by said AC electric power source, a thyristor connected between said AC electric power source and said electric heater means for switching on or off the power supply to said electric heater means, a resistor, and an inductor thermo-coupled with said electric heater means, said inductor comprising: a winding coupled to said AC electric power source through said resistor, and said winding being further coupled to the control terminal of said thyristor, and a core on which said winding is wound, said core having a Curie point of a predetermined temperature, and said core having a generally rectangular hystersis loop with a less coercive force and a high differential permeability at the magnetic field strength equal to the coercive force, whereby when said core is at temperatures below said predetermined temperature a self-induced impulse signal is present on said winding at each zero voltage point of the AC power signal and when said core is at a temperature at or above said predetermined temperature no output is present on said winding, said thyristor being turned on by the impulse present on said winding when the temperature of said core is below said predetermined temperature to couple the AC electric power to said electric heater to energize said electric heater, said thyristor being maintained in its off condition when said temperature of said core is at or above said predetermined temperature.
 9. The temperature control apparatus as claimed in claim 8, in which the core of said inductor is of one selected from the group of an alloy comprising 78-80 % Ni, Nb, Cu, Mo and Fe, an alloy comprising 50.6-54.8 mol % Fe2O3, 6.4-20.4 mol % MnO2, 24.8-43.0 mol % ZnO and additions, and an alloy comprising 12.0-17.0 mol % NiO, 33.0-38.0 mol % ZnO, 50.0 mol % Fe2O3 and additions.
 10. The temperature control apparatus as claimed in claim 8, in which the core of said inductor comprises a bar-type magnetic piece having a Curie point of said predetermined temperature, and a generally U-shaped magnetic piece having a Curie point higher than the Curie point of said bar-type magnetic piece, both of said magnetic pieces being connected and fixed to each other such that a circular closed magnetic flux loop may be formed, and said winding of said inductor is mounted on said generally U-shaped magnetic piece.
 11. The temperature control device as claimed in claim 8, in which said thyristor is an SCR, and further including a rectifying diode connected in parallel with, and in the reverse direction of, said SCR.
 12. The temperature control device as claimed in claim 8, further comprising a variable voltage dividing means coupled to said winding and coupling a portion of the output on said winding to the control terminal of said thyristor.
 13. The temperature control device as claimed in claim 8, in which said winding and said resistor are connected in series, said series connection being connected to said AC electric power source through said electric heater means.
 14. The temperature control device as claimed in claim 8, wherein said winding has an inductance L and said resistor has a resistance R (ohms), and wherein the values of L and R are chosen such that: omega L << R. 